1. Field of the Invention
This invention relates to a control apparatus and method for stabilizing optical wavelength, and in particular a control apparatus and method for stabilizing optical wavelength which is suitable for use under any external conditions.
2. Description of the Related Art
In recent years, with the development of multimedia communications services, optical transmission systems which form the backbone of communications systems are moving to higher speeds and higher capacities, and optical wavelength division multiplexing is expected to make this high performance possible.
In this optical wavelength division multiplexing, several channels are transmitted on a common transmission path by assigning plural optical signals having different wavelengths as carriers to plural signals which are to be transmitted. Therefore, to avoid inter-channel interference in optical wavelength division multiplexing, interference between optical signals on adjacent wavelengths must be avoided, and optical wavelengths must consequently be stabilized with high precision.
Two factors that cause variation of the optical wavelength output by an optical transmitter are, for example, the temperature variations of laser elements and variation of laser diode driving current.
Conventionally, the optical wavelength output from the laser was stabilized by for example controlling the temperature of the laser element, or by monitoring the wavelength of the light output from the laser element so as to control the laser element temperature, as described hereafter.
First, referring to FIG. 1, a control mode will be described for stabilizing optical wavelength by maintaining the temperature of the laser element constant (referred to hereafter as "constant temperature control" mode).
An optical transmitter shown in FIG. 1 comprises a laser module 4A in which a laser diode LD is installed together with a temperature sensor 5 and a cooling/heating element 10, a temperature monitor 6 for monitoring a laser diode temperature using the temperature sensor 5, a target value setting circuit 8 for setting a target value of the laser diode temperature, a comparator 7 for comparing a value S6 of laser temperature monitored by the laser monitor 6 with a target value S8 set by the target value setting circuit 8, and a current controller 9 for controlling a current S9 supplied to the cooling/heating element 10 based on a comparison result S7 in the comparator 7.
In the optical transmitter, the difference between the laser temperature S6 monitored by the temperature sensor 5 and temperature monitor 6, and the target value S8 set by the setting circuit 8, is detected by the comparator 7, and sent to the current controller 9 as a deviation signal S7. In the current controller 9, an output current value is determined so that the detected difference becomes 0, and the cooling/heating element 10 is driven by the determined current value. Due to this temperature control, the temperature of the laser element (in this case, the laser diode) is kept constant, and the wavelength of light output from the laser element is stabilized. A control method identical to this is disclosed in Japanese Laid-open Patent Application No. 57-186383.
Next, referring to FIG. 2, a control mode will be described wherein the optical wavelength of light output from the laser is monitored to stabilize the optical wavelength (referred to hereafter as "wavelength monitoring control" mode).
The optical transmitter shown in FIG. 2 comprises a laser module 4B in which a laser diode LD is installed together with the temperature sensor 5 and cooling/heating element 10, optical coupler 11 for splitting part of the light output from the laser diode LD, optical wavelength monitor 12 for receiving the split light and monitoring its wavelength, target value setting circuit 13 for setting a target value of optical wavelength, comparator 14 for detecting a difference between a value S12 of optical wavelength monitored by the optical wavelength monitor and the target value of optical wavelength set by the target value setting circuit 13, and a current controller 9 for controlling the current S9 to be supplied to the cooling/heating element 10 based on a deviation signal S14 output by the comparator 14.
In the above optical transmitter, the wavelength of light output from the laser module 4 is monitored by the optical wavelength monitor 12, the difference between the monitored wavelength and the target value of wavelength set by the setting circuit 13 is detected by the comparator 14, and this difference is sent to the current controller 9. In the current controller 9, the current value is determined so that the detected difference becomes 0, and the cooling/heating element 10 is driven by the determined current value. An identical control method is described in Proceedings of the General Society of 1997 of the Institute of Electronics, Information and Communication Engineers, B-10-215, p. 724. In this control mode, as it is wavelength which is being monitored, wavelength variations due to laser element temperature variations or laser diode forward current variations can be suppressed.
In general, when a laser is operated for a long period, output power fluctuations occur due to deterioration over time. For this reason, the laser diode driving current is controlled to suppress output power fluctuations by an Auto Power Control (APC) circuit, but the laser optical wavelength also varies due to this change of laser diode driving current. Therefore, in the constant temperature control mode shown in FIG. 1, the optical wavelength of the laser varies as shown in FIG. 3 together with time shown on the horizontal axis.
As shown in FIG. 3, when the laser operating time is short, a driving current IF of the laser diode LD may be considered constant. Within these limits, the output current from the current controller 9 is controlled to a fixed target value even if the laser diode temperature S6 changes due to a variation of ambient temperature. Consequently, as the cooling/heating element 10 (FIG. 1) is driven by this output current, the wavelength of optical output is stabilized in the short-term.
However if the laser operating time is long, due to the laser's deterioration with time, the laser diode driving current IF(t) may vary due to the function of the APC circuit. In this case, a wavelength fluctuation amount .DELTA..lambda. due to the laser diode driving current variation is given by the following equation (1). EQU .DELTA..lambda.=.alpha..multidot.{IF(tn)-IF(t0)} (1)
where .alpha.=conversion constant between laser diode driving current and wavelength variation, IF(t0)=laser diode driving current value at initial time t0, IF(tn)=laser diode driving current value when time tn has elapsed.
This wavelength fluctuation amount .DELTA..lambda. occurs regardless of laser diode temperature, so it cannot be corrected in this control method.
Also, when laser temperature is feedback controlled to a constant value, there is generally a considerable delay in the response (temperature variations) to manipulations (cooling or heating), and if the feedback gain is increased, the feedback loop may oscillate. Therefore, the feedback gain must be made small, but as the control error is directly proportional to the inverse of the feedback gain, the laser temperature cannot necessarily be maintained constant to a high precision. Consequently, in this control method, it is difficult to achieve highly precise stabilized control of optical wavelength.
On the other hand, in the wavelength monitoring control mode shown in FIG. 2 where the laser temperature is controlled by monitoring the optical wavelength of the light output from the laser, the optical wavelength can be stabilized even if the laser diode driving current varies. Therefore, even over a long period of time when the laser may deteriorate, the wavelength can be stabilized. Moreover, optical wavelength can be monitored rapidly and with high precision, so in this control mode, the wavelength can be stabilized with high precision.
However, in the wavelength monitoring control mode of FIG. 2, when the wavelength monitoring value is 0 or has become unstable, the current controller 9 recognizes a large difference between the optical wavelength of the laser output and the control target value, and supplies a current value to the cooling/heating element 10 to excessively heat or cool the laser element. This may cause damage or deterioration of the laser element.
Typical reasons why the wavelength monitoring value is zero or unstable are, for example, decrease of the split light led into the optical monitor 12, or instability of the optical wavelength output by the laser module 4. Therefore, for example, when a stop signal of the optical output is supplied or a loss of the optical signal occurs in the transmission path comprising the optical coupler 11 and wavelength monitor 12, or when an unstable wavelength state occurs immediately after the source voltage is switched on, the signal value output by the wavelength monitor 12 may be 0 or unstable, and temperature control may be performed leading to damage or deterioration of the laser element.
For example, when a stop signal SD is input as shown in FIG. 4, the driving current IF of the laser element LD stops, and the optical output from the laser decreases. In such a state, the output S12 of the optical wavelength monitor 12 is apparently zero wavelength, the value of the wavelength difference output from the comparator 14 is a maximum value, and the output current value from the current controller 9 is also a maximum value. As a result, the laser module 4 is excessively heated or cooled by the cooling/heating element 10, and there is a risk that this may lead to damage or deterioration of the laser element LD.